Light modulation with nematic liquid crystals is a mature technology offering a host of applications including displays, spatial light modulators, and tunable filters. These devices feature large modulation depth, good contrast ratio, low cost and ion power dissipation. One major disadvantage of nematic liquid crystals which sets a limit to the applicability of this technology is their low switching speed, in the range of milliseconds.
A more recent technology, utilizing the smectic C, phase (SmC*), surface-stabilized ferroelectric liquid crystals (SSFLCs), is now increasingly entering the field of application, offering orders of magnitude faster switching (.about.microseconds) and bistability. This class of devices is not capable of performing analog intensity or analog phase modulation in a simple straightforward configuration. Recently the ferroelectric effect has also been observed in non-chiral liquid crystals.
Another member of the chiral smectic liquid crystal (CSLC) family is the non-tilted smectic A* phase (SmA*). Although in general the SmA, phase possesses no spontaneous polarization, under the influence of an applied electric field a molecular tilt is induced in nearly linear proportion to the applied electric field. This field-induced tilt is called the electroclinic effect. Compared to SSFLC devices, electroclinic CSLCs have gray scale capability and higher modulation speed. The tilt angle induced by the electroclinic effect is smaller than that obtained with SSFLC devices; however, it is quite a considerable one, presently achieving maximum values of 15.degree.-20.degree..
Early investigations on the electroclinic effect were done using homeotropic alignment (S. Garoff and R. B. Meyer, Phys.Rev. Lett. 38, 848 (1977); S. Garoff and R. B. Meyer, Phys.Rev. A. 19, 338 (1979)). In the homeotropic alignment of CSLCs, the smectic layers are formed parallel to the confining glass plates. In order to manipulate the molecular director in the homeotropic alignment geometry, an electric field needs to be applied in a transverse direction, i.e., parallel to the substrate walls. This induces a molecular tilt in a plane which is perpendicular to the field direction. In their experiment, Garoff and Meyer detected the relative time delay between the electroclinic response of a sample and the modulating electric field signal which was applied to it. Using this measurement, in conjunction with the amplitude measurement of the electroclinic response, they were able to study the critical behavior at the smectic A*smectic C* phase transition. However, molecular tilt angles of the electroclinic effect in the homeotropic alignment were not presented in this work.
Further studies on the electro-optic properties of the electroclinic effect were performed in the surface-stabilized parallel aligned geometry (G. Andersson, I. Dahl, P. Keller, W. Kuczynski, S. T. Lagerwall, et al.) Appl. Phys. Lett. 51, 640 (1987); G. Andersson et al., Ferroelectrics 84, 285 (1988)). With this geometry it was easier to detect the electroclinic effect and tilt angles of about 6.degree. were measured. For the parallel alignment, also known as the homogeneous alignment, the planar alignment and the bookshelf alignment, the smectic layers are oriented perpendicular to the plates. The electrodes are in the plane of the plates sandwiched around the liquid crystal. An electric field is applied perpendicular to the substrates and the liquid crystal molecules rotate in the plane of the substrates. Rotation in the plane of the substrates provides analog variability of the orientation but not the magnitude of the retardance.
Tristable switching of a planar-aligned CSLC cell has been reported (I. Nishiyama et al. (1989) Jpn. J. App. Phy. 28:L2248; and A. D. I. Chandani et al. (1988) Jpn. J. App. Phy. 27:L729). The observed switching has a DC threshold and a hysteresis of the threshold voltage. The third state of such tristable cells has been linked with the presence of an antiferroelectric phase, designated SmCA*. This type of CSLC cell has been designated an antiferroelectric liquid crystal cell. The antiferroelectric phase can, for example, be generated in a SmC* material by application of an AC field across the planar-aligned liquid crystal.
A new type of chiral smectic ferroelectric liquid crystal cell called the distorted helix ferroelectric (DHF) liquid crystal cell has been described by L. A. Beresnev et al., European Patent Application No. 309774, published 1989. This type of device is similar to the planar-aligned chiral SmA* device of Andersson et al., except that it is not strongly surface-stabilized, so that the helix along the direction of the layer normal, z, is not suppressed. When the pitch of the helix (defined as the distance between identical orientations of n along the helix) is much shorter than the wavelength or wavelengths of light incident upon the device, light traversing the material sees an index of refraction given by the average orientation of the molecular directors. Application of an electric field to the DHF cell partially orients the molecular directors by an angle .PSI. to z. The angle .PSI. is dependent on the size and magnitude of the field so the DHF device operates in an analog mode similar to a SmA, device. In a DHF device there is a change in the birefringence of the material as the molecules align, which does not occur in either the SSFLC SmC* or planar-aligned SmA* device. The DHF materials, such as Hoffmann-La Roche DHF 6300, having .PSI..sub.MAX as large as .+-.37.degree. have been described. An interesting feature of DHF devices is the coupling of the change in birefringence with the rotation of the optic axis as a function of applied voltage.
Tilted smectic layer structures can be prepared in which the smectic layers are at an oblique angle to the substrate surface (S. S. Bawa et al., Appl. Phys. Lett. 57, 1398 (1990) and M. Kuwahara et al., Jpn. J. Appl. Phys. 27, 1365 (1988)). This technique is used for reducing defects in the bookshelf geometry of surface stabilized ferroelectric liquid crystal devices, in order to achieve improved properties such as higher bistability and contrast ratio. It is also used for improved alignment of nematic liquid crystals.
Fabry-Perot resonators fold the optical path within an electro-optic material and produce intensity, wavelength, and phase modulation. This technique relies on interference of waves within the cavity. By virtue of the nonlinear intensity transmission function of the Fabry-Perot resonator, a small induced phase change in the cavity produces a large intensity or wavelength modulation. Tuning the wavelengths that the filter transmits can be achieved by tuning the cavity length or the index of refraction of the cavity material. Mechanical tuning of the cavity length has been achieved by applying electric fields to piezoelectric materials, which typically have response times on the order of a few milliseconds. Index of refraction tuning has been demonstrated in liquid crystals by using nematic liquid crystals. The response time of this type of device is also on the order of milliseconds. As shown in U.S. patent application Ser. No. 07/792,284, filed Nov. 14, 1991, Fabry-Perot modulators can utilize smectic liquid crystal cells to produce phase, intensity and wavelength modulation. However, analog wavelength modulation can not be produced by a single surface-stabilized parallel aligned cell.